OVERCOMING THE METABOLIC LOAD ASSOCIATED WITH THE PRESENCE OF PLASMID DNA IN THE PLANT-GROWTH PROMOTING RHIZOBACTERIUM PSEUDOMONAS-PUTIDA GR12-2

When the broad host range plasmid vector pGSS15 was used to genetically transform the plant growth promoting rhizobacterium Pseudomonas putida GR12-2, the transformants were physiologically debilitated. It was postulated that the expression of the beta-lactamase gene of pGSS15 caused a metabolic load resulting in the impaired functioning of the bacterium. To test this hypothesis, derivatives of pGSS15 that either lack the beta-lactamase gene (pYH12-2) or in which a beta-glucosidase gene was substituted for the beta-lactamase gene (pYH124) were constructed and examined to see whether their presence also impaired the functioning of P. putida GR12-2. On the basis of growth rates, siderophore production, and the ability to stimulate canola root elongation in sterile growth pouches, neither of the newly constructed plasmids debilitated P. putida GR12-2. In addition, P. putida GR12-2 transformed with pYH124 facilitated the proliferation of the bacterium in minimal medium containing cellobiose at low temperature. This latter trait may enable P. puticia GR12-2 to persist in the soil in competition with other microorganisms.